Usukinu Yasumi (薄絹休味,Usukinu Yasumi?) is Tokiwadai Middle School student in love with Misaka Mikoto. She appears exclusively in the appended science side parts, as part of the Toaru Majutsu no Index Drama CD.

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In her first appearance before Mikoto and Kuroko, she carried her person as a nervous and timid girl who stammers a lot.

However, even before her true nature was revealed, Kuroko already felt that there was something strange about her. It is later revealed that she is the "same kind" of person as Kuroko, going all the way in trying to take Mikoto's love by following a rumor and is as noisy as Kuroko herself.

Not only that, she also enjoys being shocked by Mikoto just like Kuroko when being punished by her.

Not much is known about Yasumi, only that she entered Tokiwadai Middle School as a level 3 esper. In the story, she still a 1st year.

She apparently admires Mikoto very much and later hears about a rumor in which Mikoto wants someone stronger to protect her, and if someone could force Mikoto to go all out with her powers then they will obtain her heart.

Seeing this as an opportunity to gain Mikoto's love, and the fortuitous fact that she has a power that can counter Mikoto's she follows the rumor and decides to fight Mikoto.

After looking Mikoto the entire day, she meets up with her and Kuroko where she nervously challenges her to a battle. After much delay, Mikoto accepts her jokingly and tries to scare her off with a warning shot, however, she realizes that her lightning attack did not work on her.

Their battle is later interrupted by the arrival of the Tokiwadai Dorm Supervisor, forcing Mikoto to run away. After much searching for her, Yasumi stops by an ope cafe for a break, not knowing that the table she is using is the same one where Mikoto is hiding. Predictably, Mikoto's cover is blown, and tired of hiding and running finally fights Yasumi seriously.

Just like before, Mikoto discovers that her electric attacks are not working, to which Yasumi explains that here Side Arm (which she calls) ability prevents her from producing electricity. Believing she finally got Mikoto on the ropes, she starts to gloat, however, Terunoa's spell activates with a thunderous lightning bolt and an earthquake begins, scaring Yasumi, believing that that is the power of Misaka Mikoto (who also has no idea of what is going on but tries to use her distraction into her favor).

However, in all the mess, a billboard starts to shake and begin to collapse on top of Yasumi, who is too shocked to move. Knowing that it is all up to her, Mikoto breaks apart from Yasumi's Side Arm and uses electric powers again to use Railgun on the billboard and saving her.

In the aftermath, Mikoto discovers the true reason why Yasumi wants to fight her, and concludes much to her chagrin that she is the same type of person as Kuroko.

Yasumi calls her Level 3 ability Side Arm (サイドアーム,Saido Āmu?), though is referred to as Insulation (気力絶縁,Kiryoku Zetsuen
?, lit. "Energy Insulation") in the booklet that comes together with the CD there is no entry for Side Arm and Insulation listed instead. A possible guess is that Side Arm refers to the Telekinesis power, while the byproduct of blocking electricity is called Insulation.

A variation of Telekinesis, her ability interferes the motion of electrons and will repel electric attacks that come her way. Within her considerably large field, the movement of electrons is interfered, preventing electromagnetic phenomena to occur, such as Mikoto's Railgun and Ironsand sword. During their brief battle, this allowed Yasumi to put pressure on Mikoto's tactics. However, it will not obstruct the bioelectricity of organisms as it only applies to the outside of a body.

Since it is still a telekinetic ability, Yasumi can also lift and throw, though only on a small degree such as a chair, making it is a good counter against Electromaster type espers.

However, there is apparently a limit to how much electricity her ability can repel, as Mikoto was able to break her repulsion field near the end, not like Kamijou Touma's Imagine Breaker.